1. Field of the Invention
The present invention relates to a method for producing a semiconductor device using a crystalline thin film semiconductor.
2. Description of the Related Art
Recently, much attention is paid on a transistor constructed of a thin film semiconductor formed on a glass or quartz substrate. Such a thin film transistor (TFT) is constructed of a thin film semiconductor having a thickness of several hundreds to several thousands of angstroms (xc3x85), formed on the surface of a glass substrate or a quartz substrate (insulated gate field effect transistor).
TFTs are used in an application field such as the field of an active matrix type liquid crystal display device. An active matrix type liquid crystal display device has several hundred thousands of pixels arranged in a matrix, and TFTs are provided to each of the pixels as switching elements to realize a high quality image display. Practically available TFTs designed for active matrix type liquid crystal display devices utilize thin films of amorphous silicon.
However, TFTs based on thin films of amorphous silicon are still inferior in performance. If a higher display function is required as a liquid crystal display of an active matrix type, the characteristics of TFTs utilizing an amorphous silicon film are too low to satisfy the required level.
Further, it is proposed to fabricate an integrated liquid crystal display system on a single substrate by using TFTs to realize not only the pixel switching, but also the peripheral driver circuit. However, a TFT using an amorphous silicon thin film cannot constitute a peripheral driver circuit because of its low operation speed. In particular, a basic problem is that a CMOS circuit is unavailable from a amorphous silicon thin film. This is due to the difficulty in implementing a practical P-channel type TFT by using amorphous silicon thin film (i.e., the TFT using an amorphous silicon thin film is practically unfeasible due to its too low performance).
Another technology is proposed to integrate other integrated circuits and the like for processing or recording image data, etc., on a single substrate together with the pixel regions and the peripheral driver circuits. However, a TFT using a thin film of amorphous silicon is too inferior in characteristics to constitute an integrated circuit capable of processing image data.
On the other hand, there is a method for manufacturing a TFT using a crystalline silicon film which is far superior in characteristics as compared with the one using a thin film of amorphous silicon. The method for manufacturing TFT comprises the steps of: forming an amorphous silicon film; and modifying the resulting amorphous silicon film into a crystalline silicon film by subjecting the amorphous silicon film to heat treatment or to laser irradiation. The crystalline silicon film obtained by crystallizing an amorphous silicon film generally yields a polycrystalline structure or a microcrystalline structure.
As compared with a TFT using an amorphous silicon film, a TFT having far superior characteristics can be obtained by using a crystalline silicon film. In mobility which is one of the indices for evaluating a TFT, a TFT using amorphous silicon film yields 0.5 to 1 cm2/Vs or lower (in an N-channel TFT), but a TFT using a crystalline silicon film has a mobility of about 100 cm2/Vs or higher in an N-channel TFT, or about 50 cm2/Vs or higher for a P-channel TFT.
The crystalline silicon film obtained by crystallizing an amorphous silicon film has a polycrystalline structure. Hence, various problems arise due to the presence of grain boundaries. For example, carriers which move through the grain boundaries greatly limit the withstand voltage of the TFT. The change or degradation in characteristics which occurs in high speed operation is another problem. Further, the carriers which move through the grain boundaries increase the OFF current (leak current) when the TFT is turned off.
In manufacturing a liquid crystal display device of an active matrix type in a higher integrated constitution, it is desired to form not only the pixel region but also the peripheral circuits on a single glass substrate. In such a case, it is required that the TFTs provided in the peripheral circuit operate a large current to drive several hundred thousands of pixel transistors arranged in the matrix.
A TFT having a wide channel width must be employed to operate a large current. However, even if the channel width should be extended, a TFT using a crystalline silicon film cannot be put into practice because of the problems of withstand voltage. The large fluctuation in threshold voltage is another hindrance in making the TFT practically feasible.
A TFT using a crystalline silicon film cannot be applied to an integrated circuit for processing image data because of problems concerning the fluctuation in threshold voltage and the change in characteristics with passage of time. Accordingly, a practically feasible integrated circuit based on the TFTs which can be used in place of conventional ICs cannot be realized.
To overcome the problems concerning TFTs using a thin film of amorphous silicon or TFTs using a thin film of polycrystalline or microcrystalline silicon, a method for manufacturing a TFT using a particular region is known in the art. The method for manufacturing a TFT comprises steps of forming a region which can be regarded as a single crystal in a particular region of an amorphous silicon thin film, and then forming a TFT utilizing this particular region. By employing the method, a TFT which exhibits characteristics well comparable to those of a transistor formed on a single crystal silicon wafer (i.e., a MOS type transistor) can be obtained.
The above technology is-disclosed in JP-A-Hei-2-140915 (the term xe2x80x9cJP-A-xe2x80x9d signifies xe2x80x9cUnexamined Published Japanese Patent Applicationxe2x80x9d). In FIG. 2A, the method comprises the steps of forming a region 201 provided as a seed crystal, and then applying heat treatment to perform crystal growth from the region 201 as the seed crystal in a direction of an arrow 203 to finally crystallize a region of amorphous silicon patterned into a shape 202.
However, in FIG. 2A according to a conventional method, crystal growth occurs from a region 204 simultaneously with the crystal growth that is initiated from the region 201 in which the amorphous silicon patterned into the shape 202 is used as the seed crystal. That is, when the method of FIGS. 2A and 2B is employed, unwanted seeds of crystal growth is formed additionally in the region 204 to allow crystal growth to occur in plural modes. Thus, a polycrystalline state comprising internal crystal grain boundaries is obtained. In heat treatment, crystal growth cannot be performed within a desired area.
An object of the present invention is to provide a method which efficiently forms a region equivalent to (corresponding to) a single crystal in an amorphous silicon film provided as a starting film on a substrate having an insulating surface. Another object of the present invention is to provide a thin film transistor (TFT) that is free from the influence of grain boundaries. Another object of the present invention is to provide a TFT having a high withstand voltage and which is capable of operating a large current. Another object of the present invention is to provide a TFT whose characteristics do not undergo degradation or fluctuation with passage of time. Another object of the present invention is to provide a TFT whose performance is well comparable to that of a single crystal semiconductor.
According to one aspect of the present invention, there is provided a method comprising the steps of: forming selectively a layer of a metal element which accelerates (promotes) the crystallization of silicon in contact with the surface of the amorphous silicon film; and forming a region equivalent to a single crystal by irradiating a laser light to the amorphous silicon film while moving the laser light in the direction for increasing the area of the amorphous silicon film, wherein the laser light is irradiated while heating the amorphous silicon film.
In another aspect of the present invention, there is provided a method comprising the steps of: forming selectively a layer of a metal element which accelerates the crystallization of silicon in contact with the surface of the amorphous silicon film; patterning the amorphous silicon film in such a shape that the patterned area gradually increases from the region in contact with the layer of the metal element; and forming a region equivalent to a single crystal by irradiating laser light while moving the laser light in the direction for increasing the patterned area, wherein the laser light is irradiated while heating the amorphous silicon film. The amorphous silicon film is formed by plasma CVD, low pressure thermal CVD, etc., on a substrate having an insulating surface such as a glass substrate or a quartz substrate.
The metal element for accelerating (promoting) the crystallization of silicon is at least one selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt.
The metal layer can be formed selectively by forming a layer of the metal element on the surface of the amorphous silicon film and then patterning the resulting layer of the metal element. The layer of the metal element (which may be a layer containing the metal element) can be formed most preferably by forming a layer of nickel silicide on the surface of the amorphous silicon film by a method which comprises the steps of, coating the surface of the amorphous silicon film with a solution containing the metal element and then performing heat treatment.
In the above constitution, the step of xe2x80x9cpatterning the amorphous silicon film in such a shape that the patterned area gradually increases from the region in contact with the layer of the metal elementxe2x80x9d corresponds to a step of patterning an amorphous silicon film into a shape 102 in FIG. 1A. In FIG. 1A, the area of the shape 102 increases with an angle of xcex8 from the portion to which a layer 101 is formed in contact with the metal element.
In the above constitution, the step of xe2x80x9cforming a region equivalent to a single crystal by irradiating laser light while moving the laser light in the direction for increasing the area of the amorphous silicon filmxe2x80x9d corresponds to a step in FIG. 1B. In FIG. 1B, a laser light is irradiated while scanning (moving) in a direction of an arrow to sequentially allow the crystals to grow from the region 101 in a direction of an arrow 103 in FIG. 1A, thereby forming a region 104 equivalent to a single crystal. The laser light is, for instance, an excimer laser.
The region equivalent to a single crystal is a region which is free of internal crystal boundaries (line defects and planar defects). That is, the region equivalent to a single crystal is a monodomain region. Since point defects are present in the monodomain regions, the regions contain hydrogen or a halogen element for neutralization at a concentration of 1xc3x971017 to 5xc3x971019 cmxe2x88x923.
The metal element for accelerating the crystallization of silicon is also present at a concentration of 1xc3x971014 to 1xc3x971019 atomsxc2x7cmxe2x88x923. The concentration is defined as a minimum based on the data obtained by SIMS (secondary ion mass spectroscopy). The detection limit of SIMS at present for the metal element is 1xc3x971016 atomsxc2x7cmxe2x88x923. However, the concentration of the metal element can be approximated from the concentration of the metal element in the solution used for introducing the metal element. That is, the concentration beyond the limit of observed value by SIMS can be approximately calculated from the relation between the concentration of the metal element in the solution and the final concentration observed by SIMS for the metal element remaining in silicon film.
The region equivalent to single crystal further contains carbon atoms and nitrogen atoms at a concentration of 1xc3x971016 to 5xc3x971018 atomsxc2x7cmxe2x88x923 and oxygen atoms at a concentration of 1xc3x971017 to 5xc3x971019 atomsxc2x7cmxe2x88x923. These atoms originate from the starting amorphous silicon film formed by CVD.
According to another aspect of the present invention, there is provided a method comprising the steps of: forming selectively a layer of a metal element which accelerates the crystallization of silicon in contact with the surface of the amorphous silicon film; applying heat treatment to allow the crystals to grow in the direction of the plane of the film from the region which is in contact with the metal element; patterning the region of crystal growth such that the area gradually increases in the direction of crystal growth; and forming a region equivalent to a single crystal by irradiating a laser light to the amorphous silicon film while moving the laser light in the direction along which the patterned area increases, wherein the laser light is irradiated while heating the amorphous silicon film at 400 to 600xc2x0 C.
In the above constitution, the step of xe2x80x9capplying heat treatment to all the crystals to grow in the direction of the plane of the film from the region in contact with the metal elementxe2x80x9d corresponds to a constitution of FIG. 5B. In FIG. 5B, an amorphous silicon film 501 undergoes crystal growth in a direction of the film plane (in a direction parallel to the surface of the substrate on which the film is formed) 503 from a region 502 in which a layer of a metal element as a crystal seed is formed.
In the above constitution, the step of xe2x80x9cpatterning the region of crystal growth such that the area gradually increases in the direction of crystal growthxe2x80x9d corresponds to a step in FIG. 6A. In FIG. 6A, heat treatment is effected such that a pattern having a shape 505 is obtained, such that the area thereof gradually increases in the direction of crystal growth shown with an arrow 503.
In the above constitution, the step of xe2x80x9cforming a region equivalent to a single crystal by irradiating a laser light to the amorphous silicon film while moving the laser light in the direction along which the patterned area increasesxe2x80x9d corresponds to a step in FIG. 6B. In FIG. 6B, the laser light is scanned and irradiated in a direction of gradually increasing the patterned area 505.
To make a general classification, there are two methods for the introduction of the metal element for accelerating the crystallization.
One of the methods comprises the steps of, forming an extremely thin film of the metal on the surface of the amorphous silicon film (or on the surface of the base film formed under the amorphous silicon film) by a physical method such as sputtering or electron beam vapor deposition. In the above methods, the metal element is incorporated into the amorphous silicon film by forming a film of the metal element in contact with the amorphous silicon film. In this method, it is difficult to precisely control the concentration of the metal element to be introduced into the amorphous silicon film. Moreover, in an attempt to precisely control the quantity of the metal element to be introduced into the film by forming an extremely thin film about several tens of angstroms (xc3x85), it becomes difficult to form a film in a complete form. In this case, island-like film portions of metal element are formed on the surface of the forming surface. That is, a discontinuous layer is formed. This can be overcome by, for example, molecular beam epitaxy (MBE) and the like. However, in practice, MBE is only applicable to a limited area.
In case crystallization is effected after forming the discontinuous layer, each of the island-like regions which constitute the discontinuous layer functions as a nucleus to accelerate the crystallization. By careful observation of the crystalline silicon film obtained by the crystallization from the island-like regions, amorphous components are found to remain in a great number. This can be observed by using an optical microscope or on an electron micrograph. Otherwise, this can be confirmed through the measurements using Raman spectroscopy. It is also confirmed that the metal components remain as aggregates in a crystalline silicon film. The crystalline silicon film is finally used as a semiconductor region, However, when the metal components remain partially as aggregates, these aggregate portions function as recombination centers for electrons and holes in the semiconductor regions. These recombination centers induce particularly undesirable characteristics such as an increase in leak current of the TFT.
In contrast to the physical methods for introducing metal elements mentioned above, there is a chemical method for introducing a metal element for accelerating the crystallization of silicon. This method comprises the steps of, providing the metal element into a solution, and adding the resulting solution to the surface of the amorphous silicon film or to the surface of the base film on which the amorphous silicon film is formed by spin coating and the like. Several types of solution can be used depending on the metal element to be introduced into the amorphous silicon film. Typically, a metal compound available in the form of a solution can be used. Examples of the metal compounds usable in the solution method are shown below.
(1) In nickel (Ni), the nickel compound is at least one selected from the group consisting of nickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickel chloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate, nickel oxide, nickel hydroxide, nickel acetyl acetonate, nickel 4-cyclohexylacetate, and nickel 2-ethylhexanate. Nickel may be mixed with a non-polar solvent which is at least one selected from the group consisting of benzene, toluene, xylene, carbon tetrachloride, chloroform, ether, trichloroethylene, and Freon.
(2) When iron (Fe) is used as the catalytic element, an iron salt selected from compounds such as ferrous bromide (FeBr2.6H2O), ferric bromide (FeBr3.6H2O), ferric acetate (Fe(C2H3O2)3.XH2O), ferrous chloride (FeCl2.4H2O), ferric chloride (FeCl3.6H2O), ferric fluoride (FeF3.3H2O), ferric nitrate (Fe(NO3)3.9H2O), ferrous phosphate (Fe(PO4)2.8H2O), and ferric phosphate (FePO42H2O) can be used.
(3) In case cobalt (Co) is used as the catalytic element, useful compounds thereof include cobalt salts such as cobalt bromide (CoBr.6H2O), cobalt acetate (Co(C2H3O2)2.4H2O), cobalt chloride (CoCl2.6H2O), cobalt fluoride (CoF2.xH2O), and cobalt nitrate (Co(NO3)2.6H2O).
(4) A compound of ruthenium (Ru) can be used as a catalytic element in the form of a ruthenium salt, such as ruthenium chloride (RuCl3.H2O).
(5) A rhodium (Rh) compound is also usable as a catalytic element in the form of a rhodium salt, such as rhodium chloride (RhCl3.3H2O).
(6) A palladium (Pd) compound is also useful as a catalytic element in the form of a palladium salt, such as palladium chloride (PdCl2.2H2O).
(7) In case osmium (Os) is selected as the catalytic element, useful osmium compounds include osmium salts such as osmium chloride (OsCl3).
(8) In case iridium (Ir) is selected as the catalytic element, a compound selected from iridium salts such as iridium trichloride (IrCl3.3H2O) and iridium tetrachloride (IrCl4) can be used.
(9) In case platinum (Pt) is used as the catalytic element, a platinum salt such as platinic chloride (PtCl4.5H2O) can be used as the compound.
(10) In case copper (Cu) is used as the catalytic element, a compound selected from cupric acetate (Cu(CH3COO)2), cupric chloride (CuCl2.2H2O), and cupric nitrate (Cu(NO3)2.3H2O) can be used.
(11) In using gold (Au) as the catalytic element, it is incorporated in the form of a compound selected from auric trichloride (AuCl3.xH2O) and auric hydrogenchloride (AuHCl44.H2O).
Each of the above compounds can be sufficiently dispersed in the form of single molecules in a solution. The resulting solution is applied dropwise to the surface on which the catalyst is to be added, and is subjected to spin-coating by rotating at 50 to 500 revolutions per minute (RPM) to spread the solution over the entire surface.
This method using a solution can be considered as a method for forming a film of an organometal compound containing a metal element on the surface of a silicon semiconductor. The metal element which accelerates the crystallization of silicon is allowed to diffuse into the semiconductor the form of atoms through the oxide film. In this manner, they can be diffused without positively forming crystal nucleus, and thereby provides a uniformly crystallized silicon film. As a result, the metal element can be prevented from being partially concentrated or the amorphous component can be prevented from remaining in a large quantity.
The silicon semiconductor can be uniformly coated with an organometal compound, and then ozone treatment (i.e., treatment using ultraviolet radiation (UV) in oxygen) may be performed. In such a case, a metal oxide film is formed, and the crystallization proceeds from the metal oxide film. Thus, the organic matter can be favorably oxidized and removed by volatilization in gaseous carbon dioxide.
In case spin coating of the solution is effected by rotating at a low speed only, the metal component that is present in the solution on the surface tends to be supplied onto the semiconductor film at a quantity more than is necessary for the solid phase growth. Thus, after rotating at a low revolution rate, the spin coating is effected by rotating the substrate at 1,000 to 10,000 RPM, typically, 2,000 to 5,000 RPM. The organometal compound that is present in excess can be spun off by rotating at high rate, so that the metal component can be supplied at an optimum quantity.
The quantity of the metal component to be introduced into the silicon semiconductor can be adjusted by controlling the concentration of the metal component in the solution. This method is particularly useful, because the concentration of the metal element to be finally introduced into the silicon film can be accurately controlled. In the method of introducing the metal element using the solution, a continuous layer can be formed on the surface of the semiconductor (or on the surface of the undercoating thereof) without forming island-like regions of metal particles for the crystallization. Then, a uniform and dense crystal growth can be effected by a crystallization method with heat treatment or with irradiation of laser light.
In the foregoing, an example of using a solution has been described, but a similar effect as the one above can be obtained by forming the film by CVD using a gaseous metal compound, and particularly, a gaseous organometal compound. However, this method using CVD is disadvantageous in that it is not as simple as the one using a solution.
The method for forming the layer by sputtering and the like as described above can be denoted as a physical method. The method using a solution in forming a layer containing a metal element for accelerating the crystallization of amorphous silicon can be considered as a chemical method. The physical method can be considered as a non-uniform anisotropic crystal growth method using metal elements, whereas the chemical method can be considered as a method for uniform (isotropic) crystal growth.
In the method for manufacturing a semiconductor as described above, the laser light is irradiated in a direction of gradually increasing the area of a region in which the seeds of crystal growth are formed. In this manner, a uniform crystal growth can be effected to form a region equivalent to a single crystal.
Further, the laser light is irradiated to the amorphous silicon film which is patterned such that the area gradually increases from the region in which the seeds of crystal growth are formed to accelerate the crystallization while heating and scanning the laser light in a direction for increasing the area of the amorphous silicon film. In this manner, a uniform crystal growth can be effected to form a region equivalent to a single crystal.
Also, by patterning a silicon film obtained by crystal growth in a direction in parallel with the substrate in such a manner that the area thereof gradually increases, and further by irradiating a laser light while heating and scanning it in a direction of gradually increasing the area of the patterned film, a region equivalent to a single crystal can be obtained, because the crystal growth is allowed to occur in a single mode.